Medical implant

ABSTRACT

A medical implant such as a heart stimulator has a detector which detects an extracorporeally generated interrogation signal for at least one predetermined working parameter of the medical implant. The interrogation signal is generated by an interrogation signal device capable of only unidirectional communication to the medical implant. The implant includes a response signal generator which generates an extracorporeal detectable response signal, detectable, for example, by a stethoscope, which indicates only if the interrogated working parameter has a satisfactory value or a non-satisfactory value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a medical implant of the type whichgenerates an extracorporeally detectable signal in response to aninterrogation signal, as well as to an interrogation signal generatorfor generating such an interrogation signal.

2. Description of the Prior Art

In a normal follow-up for a pacemaker patient many different workingparameters of the pacemaker are tested, e.g. battery status, stimulationthreshold, electrode lead impedance and others. The follow-up is made ina hospital at least once per year and the physician uses abi-directional communication programmer that communicates with thepacemaker by radio waves. The requested information is received by theprogrammer and analyzed by the physician, which is a difficult andtime-consuming job. The programmer is quite expensive and may not alwaysbe a part of the ordinary equipment for every small-sized hospital.Battery tests can also be performed by putting a magnet over thepacemaker that changes the stimulation rate in dependence of the batterystatus. This rate can be seen on an ECG machine or by listening for orfeeling the pulse. Devices used for performing battery tests are forinstance disclosed in U.S. Pat. No. 54,390,020 and U.S. Pat. No.4,416,282.

In U.S. Pat. No. 4,390,020 a programmable pacemaker is disclosed capableof operating in several stimulating modes, having battery poweredstimulating means and stimulating mode selector means. Sensing andevaluating means, which can be activated externally by means of amagnet, for example at a medical examination, monitor the terminalvoltage of the battery and cause the pacemaker via the stimulatingselector means to change operation from a first stimulating mode with aprogrammed stimulation rate to a mode with a fixed stimulation rate,when the terminal voltage decreases below a first threshold value andoperate in a second predetermined stimulating mode at a fixedstimulation rate when the terminal voltage decreases below the first aswell as a lower second threshold value.

U.S. Pat. No. 4,416,282 discloses a similar pacemaker which alsoincludes sensing and evaluating means for monitoring of the batterycapacity with regard to two battery depletion levels. The stimulationrate automatically decreases with the decreasing of the battery capacitybelow the depletion levels.

In U.S. Pat. No. 4,488,555 a battery condition warning system for amedical implant is known. The warning system generates an audible alarmto warn the patient of an impending battery failure.

In U.S. Pat. No. 4,614,192 an implantable cardiac defibrillator isdisclosed, providing, upon magnet-type interrogation, an audibleindication to verify the status of the implanted device. To enable thedefibrillator to deliver a defibrillating pulse a control circuit mustbe placed in an active state. To place it in an active state a ringmagnet is used to toggle a status flip-flop that emits an enablingsignal to the control circuit. At the same time, an audio oscillator isenergized by the output signal from the status flip-flop and from a ratecircuit, enabling the audio oscillator to emit sounds synchronous withthe heart beat if a bipolar electrode is properly positioned within theheart and to emit a continuous tone if the defibrillator is inactive andproperly positioned. Absence or presence of an audible tone indicateswhether the probe is properly lodged about the right ventricle. Theaudible indication in the defibrillator disclosed in U.S. Pat. No.4,614,192 is generated in response of a magnet interrogation andreflects the status of the implant at the time the interrogation ismade, i.e. the measurement procedure is performed at that time.

One drawback for many of these known solutions is that some kind of moreor less complicated technical equipment is needed, e.g. a programmer oran ECG-machine, not always available in smaller clinics.

Another drawback is that before the implant can respond to aninterrogation from an external device, often time-consuming tests has tobe performed by the implant.

SUMMARY OF THE INVENTION

An object of the Invention is to provide a medical implant capable ofimmediately generating an response signal that is easy to recognizeoutside the body by e.g. a stethoscope, and that reflects the presentstatus of at least one predetermined working parameter of the implant.Another object is for the response signal indicates if the workingparameter has a satisfactory value or not.

One further object of the invention is to provide an interrogationsignal-generating device adapted to work with a medical implant whichachieves the above objects.

The above objects are achieved in a medical implant in accordance withthe principles of the present invention which contains a predeterminednumber of working parameter status registers, each containing updateddata representing a first state or a second state of a workingparameter, the first state indicating that the working parameter has asatisfactory value and the second state indicating that the workingparameter has a non-satisfactory value and wherein, in response todetection of an interrogation signal inquiring about a status of aparticular working parameter, the medical implant generates a responsesignal which only identifies the content of the status register for theworking parameter which was the subject of the interrogation signal, andthe response signal is extracorporeally detectable.

Since the response signal indicates only whether the inquired-aboutworking parameter has a satisfactory value or a non-satisfactory value,it can be very simply extracorporeally detected, such as by astethoscope or tactilely, depending on the type of response signal whichis generated. Since the response signal has only one of two possiblestates, there is no need to analyze the signal beyond identifying itsstate.

The above objects are also achieved in an interrogation signalgenerator, which operates with a medical implant as described above,which is adapted only for one-way signaling to the medical implant. Theinterrogation signal generator contains only a signal generator, anenergy source, a control unit and a selection unit which is used toactivate the signal generator to generate an appropriate interrogationsignal regarding a particular working parameter.

DESCRIPTION OF THE DRAWING

The single FIGURE is a schematic block diagram of a medical implant andan interrogation signal generator constructed and operating inaccordance with the principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the figure the medical implant 2 and the interrogation signalgenerating device 18 are shown in a block diagram. The medical implant2, e.g. a pacemaker or a defibrillator, contains a first control unit 4connected to a response signal generator 6, an interrogation signaldetector parameter status registers 14 and a test unit 16. As iswell-known to those skilled in the art the medical implant 2 alsoincludes inter alia an energy source, one or many electrode leads forapplying stimulation pulses to the tissue etc. For simplicity, however,these features are not described in detail neither in the FIGURE nor inthe description since they are not a part of the invention and theirfunctions are well known to persons skilled in the art.

The interrogation signal generating device 18 includes only a signalgenerator 20 adapted to generate an interrogation signal 12, an energysource 22, selection unit 24 with a predetermined number of activationbuttons 28, 30, 32, 34 (four in the FIGURE) and a control unit 26. Eachactivation button 28, 30, 32, 34 represents at least one workingparameter of the medical implant 2. The working parameters could be e.g.the battery status, the status of lead impedance or the status of astimulation threshold. One of the activation buttons 28, 30, 32, 34represents the overall status of all working parameters together.

The medical implant 2 and the interrogation signal generating device 18work together in the following way: The person who is going tointerrogate the implant 2 places the device 18 on the skin of a patientabove the implant 2, and presses one of the activation buttons 28, 30,32, 34 causing the control unit 26 to activate the signal generator 20to generate an interrogation signal 12. The working parameterrepresented by the pressed activation button is univocally identified bythe generated interrogation signal 12.

The interrogation signal is preferably a radio-wave signal having afrequency in the range 2-8 kHz. A predetermined communication protocolis used enabling said univocal identification of the interrogationsignal 12. According to an alternative embodiment the interrogationsignal is a magnetic signal 12. Interrogation is then made by placing adevice capable of generating magnetic field of a predetermined kind,e.g. a magnet, on the skin close to the implant.

The interrogation signal 12 is detected by the interregation signaldetector 10 in the medical implant 2. According to a preferredembodiment the telemetry coil, normally used for ordinary communicationbetween an external programming device and a medical implant, is usedfor detecting the interrogation signal 12. Since this technique is wellknown to those skilled in the art it need not be described herein. Thedetected interrogation signal 12 is applied to the control unit 4 whichaddresses the working parameter status register 14 that matches therequested working parameter. The content of the addressed statusregister is read by the control unit 4 which then activates the responsesignal generator 6 to generate a response signal 8 which is detectableoutside the body of the implant wearer.

According to a first preferred embodiment the response signal 8 is anacoustic signal generated by an acoustic signal generator, e.g. apiezoelectric crystal. The frequency of the generated acoustic tonecould be in the range of 100-1900 Hz, preferably 1000-1900 Hz where 1400Hz is a preferred value. The tone should be strong enough to be able tobe detected by a stethoscope placed on the skin close to the implant.When an interrogation signal 12 is detected by the interrogation signaldetector 10 the control unit 4 identifies the working parameter statusregister requested by the interrogation signal 12. The value,“satisfactory” or “non-satisfactory”, in the requested register is readout by the control unit 4 and the response signal generator 6 isactivated to generate the response signal 8, in this embodiment anacoustic signal. The response signal generator is preferably activeduring 5 minutes and generates a response signal every 15^(th) second.The response signal 8 representing the states “satisfactory” or“non-satisfactory” can of course be chosen in many different ways, e.g.“satisfactory” could be represented by five short tones followed by fivelong tones, and “non-satisfactory” could be represented by the sequencethree short tones, three long tones and three short tones.

According to a second preferred embodiment the response signal 8 isrepresented by a predetermined change of the stimulation frequency. Thischange in frequency can be palpated directly, detected by a stethoscopeor studied on a print-out from an ECG-equipment.

When, according to this second preferred embodiments, an interrogationsignal 12 is detected by the interrogation signal detector 10, thecontrol unit 4 changes, if necessary, the pacing mode to V00, that isventricular stimulation with no sensing and no inhibition possible, andthe stimulation rate to a predefined rate related to the state of theinterrogated working parameter. The state “satisfactory” could e.g. berepresented by a stimulation rate per minute of 100 and the state“non-satisfactory” could then be represented by a rate of 80.

If the interrogation signal 12 is a radio-wave signal this predefinedrate preferably could last for e.g. 32 pacing intervals and if theinterrogation signal 12 is a magnetic signal, as long as the magneticfield is present.

By continuously updating the working parameter status registers 4 theresponse signal can be generated almost immediately because notime-consuming tests of the different working parameters has to beperformed. The working parameters of the medical implant 2 could be, asindicated above, e.g. the battery status, the status of lead impedanceor the status of the stimulation threshold. One of the activationbuttons 28, 30, 32, 34 represents the overall status of all workingparameters together.

The most commonly used battery in modern pacemakers is thelithium-iodine battery. As current is drained from the battery anincrease in the internal impedance of the battery occurs. Since the rateof increase in battery impedance versus time at any specific batterycurrent drain is known, measured battery current drain and batteryimpedance allow prediction of remaining device longevity. In practice,the internal impedance is measured at regular intervals, e.g. every24^(th) hour, and compared to a predetermined threshold representing animpedance value corresponding to the recommended replacement time (RRT),being e.g. 2 years. The battery status is given the state “satisfactory”if the RRT is more than e.g. 2 years and the state “non-satisfactory” ifless than 2 years.

A very vital part of a pacemaker system is the electrode lead connectingthe pacemaker to the inside of the heart. The electrode -lead isinserted into the heart via e.g. a great vein and comprises anelectrical lead insulated by e.g. silicone. The function of an electrodelead can be tested, by measuring of the lead impedance. If the leadimpedance is decreased, it can be caused by a breakdown or damage in theinsulation of the lead, and if the lead impedance is increased it can becaused by a break or damage of the electrical lead. The lead impedancecan be measured e.g. by an lead impedance scanning system disclosed inU.S. Pat. No. 4,899,750. In this system the voltage difference over asample capacitor, before and after the delivery of a pacing pulse, isused in an equation to calculate the lead impedance. If the leadimpedance is in the range of e.g. 750+/−500 Ohm the status of leadimpedance is given the state “satisfactory” and if outside said rangethe state “non-satisfactory”.

For the stimulation threshold, for the ventricle and/or the atrium, thestate is given the value “satisfactory” if the threshold is below apredetermined value, e.g. 3 Volts and “non-satisfactory” if thethreshold is above said value. It is of course only possible to test thestimulation threshold if some kind of automatic search for thestimulation threshold can be performed, e.g. according to theAUTOCAPTURE™-algorithm, at regular intervals.

All the values used to determine if the state is “satisfactory” or“non-satisfactory” for the working parameters can of course beindividually set in dependence of the circumstances.

As indicated above one of the working parameter status registersreflects the combined status of all working parameters in the way thatif any of the other working parameters is in the“non-satisfactory”-state the combined status will be “non-satisfactory”.When the interrogation signal 12 is a magnetic signal, in accordancewith the above-mentioned alternative embodiment, the response signal 8reflects the content of the register with the combined status.

As indicated above the values stored in the working parameter statusregisters are updated continuously as a result of tests performed bysaid the test unit 16.

It is apparent to those of skill in the art that working parametersother than the above described can be used, e.g. if a certain level fora predetermined parameter is exceeded more than a predetermined numberof times the state is set to “non-satisfactory”. In general, if apredetermined event occurs (related to the heart or the pacemaker), thatnot fulfills the “satisfactory” criteria, the state for that workingparameter is set to “non-satisfactory”.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

What is claimed is:
 1. A medical implant comprising: means foradministering a medical therapy having a plurality of working parametersassociated therewith; a plurality of working parameter status registers,each of status registers containing a continually updated statusindicator for one of said working parameters, said status indicatorhaving either a first state indicating that the working parameterassociated with the status registers has a satisfactory value, or asecond state indicating that the working parameter associated with thestatus register has a non-satisfactory value, said status indicatorhaving no other state; a detector adapted to receive an extracorporeallygenerated interrogation signal regarding one of said working parameters,and emitting a detector output signal identifying said one of saidworking parameters; a control unit supplied with said detector outputsignal and connected to each of said working parameter status registers,said control unit addressing the status register for said one of saidworking parameters and reading out the status indicator thereof; and aresponse signal generator connected to said control unit and suppliedwith said status indicator, said response signal generator emitting anextracorporeally detectable response signal dependent on the statusindicator supplied by said control unit, said response signal having afirst response signal state corresponding to said first state of saidstatus indicator and a second response signal state corresponding tosaid second state of said status indicator, and no other response signalstate.
 2. A medical implant as claimed in claim 1 comprising a test unitfor monitoring said plurality of working parameters and being connectedto said plurality of status registers and automatically updating therespective status registers at predetermined intervals.
 3. A medicalimplant as claimed in claim 1 wherein said plurality of workingparameters include a working parameter identifying an overall status ofall of said plurality of working parameters.
 4. A medical implant asclaimed in claim 1 wherein said response signal generator generates anacoustic signal as said response signal.
 5. A medical implant as claimedin claim 1 wherein said response signal generator comprises anstimulation pulse generator which generates stimulation pulses as a partof said medical therapy and wherein said response signal generatorfurther comprises electrode leads adapted for delivering saidstimulation pulses to a heart, said stimulation pulses having afrequency, and wherein said response signal comprises a signal having apredetermined change of said stimulation frequency.
 6. A medical implantas claimed in claim 1 wherein said response signal generator generates astethoscope-identifiable signal as said response signal.
 7. A medicalimplant as claimed in claim 1 wherein said response signal generatorgenerates a magnetic signal as said response signal.